1. SGT1 Polypeptides
SGT1 is known as a suppressor allele of skp1 mutant.
Chung et al., 2006 report that SGT1 plays a crucial role in developmental processes. SGT1 has unique domains necessary for protein functions: tetratricopeptide repeat domain (TPR), CHORD and SGT1 motif (CS) and SGT1-specific motif (SGS motif). The TPR domain has been known to mediate protein-protein interactions among multicomplex proteins functioning as chaperone, cell cycle, transcription, or protein transport complexes. For example, the TPR domain of SGT1 was shown to bind to heat-shock protein 70 (HSP70). However, the CS domain of SGT1 is similar to the one in the human p23 protein, which is known to interact with HSP90 and participate in the folding of different regulatory proteins.
2. CLC-pKG Polypeptides
In both prokaryotic and eukaryotic organism, anion channels/transporters appear as key players in the control of metabolism, in the maintenance of electrochemical gradients and in signalling pathways leading to adaptation to abiotic and biotic environmental stresses. In plants, they contribute to various physiological functions such as control of stomatal movements regulating gas exchanges in leaves, plant-pathogen interaction, root xylem loading, compartmentation of metabolites and coupling with proton gradients (reviewed in De Angeli et al. 2007 Phil. Trans. R. Soc. B 2009 364, 195-201). Anion channel activities and associated regulation mechanisms have been characterized primarily using electrophysiological techniques. They were reported in all plant membranes including the plasma membrane, tonoplast, endoplasmic reticulum, mitochondria and chloroplasts, plasma membrane channels being by far the best characterized compared to those located on other membranes. In model plants such as rice and Arabidopsis there are up to seven genes encoding CLCs which are spread in over two distinct subfamilies (Marmagne et al. 2007, Journal of Experimental Botany, Vol. 58, No. 12, pp. 3385-3393). One of such subfamilies comprising the AtCLCe and AtCLCf proteins is close to the prokaryotic CLC, altogether belonging to the Chloride Channel Prokayotic Group while the other class is closer to the eukatyotic CLCs. The Arabidopsis thaliana AtCLCf protein is reported to have similar subcellular distribution and presumably similar function as those CLCs of Synechocystis CLC, considered to represent the ancestor precursor of plant chloroplast.
Nitrate and malate represent the majority of anions in a plant cell. Nitrate is a nutrient but can act as a signalling molecule as well. Plants have a sophisticated nitrate uptake system involving both low- and high-affinity transporters, nitrate is next either transported through the xylem to enter into the cellular metabolism or is stored locally. Cells assimilate nitrate via the nitrate reductase pathway or store it in the tonoplast, and a dynamic balance exists between cytosolic and vacuolar nitrate levels, regulated by uptake of extracellular nitrate, storage in the vacuole and anabolism. The discovery of the chloride channel (CLC) family allowed unravelling the mechanism of proton/nitrate exchange between tonoplasts and cytosol. Determination of subcellular localization, expression patterns, and characterization of knockout mutant phenotypes, gave insight in the physiological role of CLC proteins. Phenotypic analyses showed that cica-1 and cica-2 mutant plants have a reduced nitrate compared to that of wild-type in root and shoot tissues. Also cicc and cice mutants showed lower nitrate levels compared to control plants. An overview of the art is provided by De Angeli et al., Phil. Trans. R. Soc. B 364, 195-201, 2009; and the references cited therein. However, still little is know about the precise role of CLC proteins, and the effect of overexpressing CLC genes on plant phenotypes.
3. HD-Hydrolase-Like Polypeptides
The HD domain comprises a sequence of two small amino acids, followed by two hydrophobic amino acids, a histidine and an aspartic acid, again a hydrophobic amino acid, a small amino acid and a charged amino acid. Because of its weak sequence conservation, it was only recently discovered (Aravind and Koonin, Trends in Biochemical Sciences, 23: 469-472, 1998). The domain is reportedly present in metal-dependent phosphohydrolases, including nucleic acid polymerases and helicases (Aravind and Koonin, 1998). Although the HD-domain comprising proteins are predicted to exhibit phosphohydrolase activity and appears to be involved in nucleic acid metabolism or in signal transduction (Galperin et al., J. Mol. Microbiol. Biotechnol. 1, 303-305, 1999), the precise biological role of the HD domain remains to be elucidated.